The present invention relates to an automatic focus-detecting apparatus and method thereof. More particularly, this invention relates to an automatic focus-detecting apparatus applied in image input equipment having image pickup device used for a video camera or a digital camera.
As an automatic focus-detector for a digital camera, there has been known a detector using a mountain climbing servo system (xe2x80x9cAuto-focus adjustment for a television camera in the hill climbing servo systemxe2x80x9d, Technical Report by NHK, 1965, Vol. 17, No. 1, Consecutive Vol. No. 86, Page 21).
The mountain climbing servo system is realized by successively scanning and reading the image signals from image pickup device. High-frequency components included in the image signals are then extracted and integrated. This process is repeated while changing a focusing point on the image pickup device. A point at which the integrated output is the maximum is determined as the focus point. Since the focus point is detected with the help of the image pickup device and the optical system for the device, there is no need to provide any other device or optical system for detecting the focus point. Thus, the mountain climbing servo system is an excellent system. Therefore, this system is still used in circuits in which filters for extracting high-frequency components included in an image signal or integrators for integrating the outputs from such filters are digitized.
FIG. 13 is a block diagram showing an example of an automatic focus-detecting circuit based on the conventional technology with the mountain climbing servo system employed therein. In this figure, the reference numeral 301 shows a HPF (High-pass filter) which passes therethrough only high-frequency components included in input image data. Herein, output of the HPF 301 may have a positive or a negative polarity. The reference numeral 302 shows an ABS circuit (Absolute value circuit) which computes an absolute value of the output of the HPF 301. The reference numeral 303 shows a base clipping circuit which removes the output lower than a prespecified level from the output of the ABS circuit 302. The reference numeral 304 shows an integrator which integrates the output of the base clipping circuit 303 foreach image. The reference numeral 305 shows a computing circuit which temporarily stores therein the output of the integrator 304, and compares the output of the integrator 304 at a plurality of locations to determine a focus.
Operation of the automatic focus-detecting circuit in FIG. 13 is explained below. The HPF 301 extracts high-frequency components from image data input at each lens position (focusing point) when the lens is shifted to output the high-frequency components to the ABS circuit 302. The ABS circuit 302 obtains an absolute value of the output (this output may have a positive or a negative polarity) of the HPF 301 and outputs the absolute value to the base clipping circuit 303.
The base clipping circuit 303 removes components lower than the prespecified level, namely removes the components which is most probably the noise from the output of the ABS circuit 302 and outputs the rest of the components to the integrator 304. The integrator 304 integrates components included in a preset focal-point detection range in the image data of the output of the base clipping circuit 303, and outputs the integrated component to the computing circuit 305 for each image. The computing circuit 305 compares the output of the integrator 304 at each lens position (focusing point) with each other, and outputs the position at which the value is the maximum as a focus point.
In the conventional type of automatic focus-detecting circuit, however, malfunction may occur with an image including a highly bright object like a light source because integrated output becomes the maximum not at a point at which focussed but at a point at which the focus does not match. In an ordinary image, when an object is out of focus and vague, the output of the image signal in that portion becomes weak. Therefore, the extracted high-frequency component in each pixel becomes smaller, and hence the integrated output becomes smaller than that of the focused point. However, in an image including a highly bright object, even if the object is out of focus, the output is not lower than that of the focused point because an image signal of the such an object is saturated or close to saturation. As a result, the extracted high-frequency component in each pixel does not become smaller. The reason behind this is that the integrated output for the portion of the highly bright object being out of focus and vague is larger than that for a focused point by the defocused amount.
The present invention has been made in light of the problems described above. It is an object of this invention to provide an automatic focus-detecting apparatus and method thereof enabling detection of a focus with high precision even when a highly bright object is included in the image.
According to a first aspect of this invention, a high-frequency component extracting unit extracts a high-frequency component included in the picked-up image, the first clipping unit removes a component equal to or less than the first threshold value from the high-frequency component and outputs the rest of the high-frequency component, a second clipping unit removes a component equal to or less than the second threshold value which is larger than the first threshold value from the high-frequency component and outputs the rest of the high-frequency component, a first integrating unit integrates the output of the first clipping unit, a second integrating unit integrates the output of the second clipping unit, and a computing unit detects the focus point according to the integrated values obtained in the first and the second integrating unit. Therefore, the focus point can be detected with high precision even for an object containing a highly bright object.
According to a second aspect of this invention, a high-frequency component extracting unit extracts a high-frequency component included in the picked-up image, a first clipping unit removes a component equal to or less than a first threshold value from the high-frequency component to output the rest of the high-frequency component, a second clipping unit removes a component equal to or less than a second threshold value which is larger than the first threshold value from the high-frequency component to output the rest of the high-frequency component, a first integrating unit integrates the output of the first clipping unit, a second integrating unit integrates the output of the second clipping unit, a highly bright object detecting unit detects a high-brightness component in a picked-up image, and a computing unit detects the focus point according to the integrated values obtained in the first and second integrating unit and the result of detection in the highly bright object detecting unit. Therefore, it is possible to detect the focus point with high precision even for an object containing a highly bright object.
According to a third aspect of this invention, a high-frequency component extracting unit extracts a high-frequency component included in the picked-up image, a clipping unit removes a component equal to or less than a first threshold value from the high-frequency component and outputs the rest of the high-frequency component, a comparing unit outputs a comparison signal when the high-frequency component is equal to or higher than a second threshold value which is larger than the first threshold value, an integrating unit integrates the output of the clipping unit, a counter counts the output of the comparing unit, and a computing unit detects the focus point according to the integrated value obtained in the integrating unit and the counted value obtained in the counting unit. Therefore, it is possible detect the focus point with high precision even for an object containing a highly bright object.
According to a forth aspect of this invention, a high-frequency component extracting unit extracts a high-frequency component included in the picked-up image, a clipping unit removes a component equal to or less than a first threshold value from the high-frequency component and outputs the rest of the high-frequency component, a comparing unit outputs a comparison signal when the high-frequency component is equal to or higher than a second threshold value which is larger than the first threshold value, an integrating unit integrates the output of the clipping unit, a counting unit counts the output of the comparing unit, a highly bright object detecting unit detects a high-brightness component in the picked-up image, and a computing unit detects the focus point according to the integrated value obtained the integrating unit, the counted value obtained in the counting unit, and the result of detection in the highly bright object detecting unit. Therefore, it is possible to detect the focus point with high precision even for an object containing a highly bright object.